Roof bolting is used as the primary means of roof support in underground mines using the room and pillar mining method, currently 90% of U.S. underground coal mines. Typically, four to six foot deep holes are drilled vertically in the overlying rock strata. These holes are normally 1 to 13/8 inch in diameter and spaced on a four foot square grid. Steel rock bolts or roof bolts are inserted in these holes and either grouted in the hole for essentially their full length or provided with an expanding anchor at the upper end and a roofplate and bolthead at the lower end. In the latter case the bolt is normally tensioned to one half of the yield strength of the bolt, as provided in the Code of Federal Regulations, Title 30, Part 75.200. The tensioned connection thus formed between the rock which houses the anchor, and the roof surface at the other end of the bolt, renders the roof structure much more competent and self supporting.
It is evident that the properties of the rock stratum receiving the anchor are a crucial factor in determining the quality of the roof support and hence the safety of the mine.
The method used to date to ascertain the quality of the rock at the anchorage horizon consists of installing a special roof bolt equipped with a pulling collar, fitting this bolt with a hydraulic jack and an extensometer, and applying additional tension to the bolt in increments by means of the hydraulic jack. Bolt displacement is then noted on the extensometer for each load increment added. The load at which the displacement increment per unit load increment, i.e., the derivative of displacement with respect to load, increases materially, which normally occurs relatively abruptly as load is added, is considered to be the maximum capacity of the anchorage stratum.
Obviously, the method just described is time consuming, a fact which is especially disadvantageous in a coal mining situation, where production time pressures are extreme. Furthermore, the method tends to destroy the anchorage of the bolt tested, which can therefore not be relied upon for contribution to roof support. It has also been found that in many coal mines the properties of the rock immediately surrounding the anchor are highly variable, even between neighboring bolts, so that performing a few pull tests as described may not be representative of an extensive roof area.
To date there has been no simple, reliable and convenient method or means available to ascertain rock quality for each bolt installed.
This invention is accordingly directed to a novel method and novel means for detecting the compressive strength of the rock immediately surrounding a roof bolt anchor.
A principal object of the invention is to provide a practical and convenient means of determining rock strength, using a modification and augmentation of the equipment means already in use to drill the rock bolt hole and tension the rock bolt. The novel method and means to detect the rock compressive strength utilize the development of an indication of rock compressive strength on each bolt installed during the normal bolt tightening cycle, so that no machine operation time or machine operator effort is required beyond the time and effort presently used to install each roof bolt. Knowledge concerning the anchorage strata thus made available for each bolt installed, as it is installed, can accordingly be used to modify the roof support plan if needed. For example, steps such as closer bolt spacing, different bolt length, different anchor types, and the like can be taken. A material increase in mine safety is anticipated through the use of this invention.
An additional object of the invention is to obtain reliable and accurate means of measurement and control of the torque delivered to the rock bolt head during the installation process.
A further additional object of the invention is to obtain a reliable and accurate means of maintaining a relationship between tension developed in the bolt and torque applied to the bolt head. The reliability and accuracy or torque to tension relationship is achieved primarily by providing special controlling means to maintain a constant low thrust to the rock bolt head during the rock bolt tightening process.
Further objects and advantages of the invention will become apparent from a consideration of the drawings and ensuing description thereof.
Similarly it will become apparent from the detailed description of the invention that useful application is not limited to underground roof bolting. Examples of additional fields of application include, but are not limited to, determination of rock strength in exploration and production drilling activity, on the surface as well as underground; determination of wood strength in structural members as well as in living trees; determination of concrete strength in structures such as dams; and determination of strength and characterization of materials such as coal in situ.